Inorg. Chem. 2002, 41, 4981−4983
Hydrolytically Active Tetranuclear Nickel Complexes with Structural Resemblance to the Active Site of Urease Ha˚kan Carlsson,† Matti Haukka,‡ and Ebbe Nordlander*,† Inorganic Chemistry, Center for Chemistry and Chemical Engineering, Lund UniVersity, Box 124, SE-221 00 Lund, Sweden, and Department of Chemistry, UniVersity of Joensuu, Box 111, FIN-80101 Joensuu, Finland Received July 2, 2002
Reaction of the new asymmetric ligand 2-(N-isopropyl-N-((1methylimidazolyl)methyl)aminomethyl)-6-(N-carboxylmethyl-N-((1methylimidazolyl)methyl) aminomethyl)-4-methylphenol (ICIMP) with nickel perchlorate and diphenylacetic acid leads to the formation of tetranuclear nickel complexes, whose crystal structures reveal that they consist of dimers of dimers in which each Ni2 unit has a coordination environment that is similar to the active site of urease. One complex has been shown to coordinate urea and catalyze the hydrolysis of an organophosphate monoester. Urease, which hydrolyzes urea to ammonia and carbamic acid, is an important enzyme in both agriculture and medicine.1,2 It was the first enzyme to be crystallized3 and also the first enzyme that was shown to contain nickel.4-6 Several crystal structures of urease from two different organisms have recently been published.7-10 These reveal that the active site of the enzyme contains two nickel atoms which are bridged by a carbamylated lysine (Figure 1). A number of proposals for the mechanism of urease have been put forth,1,5,10-12 and studies of model complexes for * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: +46 46 222 44 39. Tel: +46 46 222 81 18. † Lund University. ‡ University of Joensuu. (1) Karplus, P. A.; Pearson, M. A.; Hausinger, R. P. Acc. Chem. Res. 1997, 30, 330-337. (2) Kolodziej, A. F. Prog. Inorg. Chem. 1994, 41, 493-597. (3) Sumner, J. B. J. Biol. Chem. 1926, 69, 435-441. (4) Dixon, N. E.; Gazzola, C.; Blakeley, R. L.; Zerner, B. J. Am. Chem. Soc. 1975, 97, 4131-4133. (5) Lippard, S. J. Science 1995, 268, 996-997. (6) Maroney, M. J. Curr. Opin. Chem. Biol. 1999, 3, 188-199. (7) Jabri, E.; Carr, M. B.; Hausinger, R. P.; Karplus, P. A. Science 1995, 268, 998-1004. (8) Pearson, M. A.; Michel, L. O.; Hausinger, R. P.; Karplus, P. A. Biochemistry 1997, 36, 8164-8172. (9) Benini, S.; Rypniewski, W. R.; Wilson, K. S.; Miletti, S.; Ciurli, S.; Mangani, S. Structure 1999, 7, 205-216. (10) Benini, S.; Rypniewski, W. R.; Wilson, K. S.; Ciurli, S.; Mangani, S. JBIC 2001, 6, 778-790. (11) Dixon, N. E.; Riddles, P. W.; Gazzola, C.; Blakeley, R. L.; Zerner, B. Can. J. Biochem. 1980, 58, 1335-1344. (12) Ciurli, S.; Benini, S.; Rypniewski, W. R.; Wilson, K. S.; Miletti, S.; Mangani, S. Coord. Chem. ReV. 1999, 190-192, 331-355.
10.1021/ic025839p CCC: $22.00 Published on Web 09/11/2002
© 2002 American Chemical Society
Figure 1. Schematic depiction of the structure of the active site of Bacillus pasteurii urease.9
the active site of the enzyme have received new impetus by the structural determination of urease.13-21 In order to prepare structural and functional mimics of urease and use these to investigate its mechanism, we have synthesized a new phenolate-based polydentate ligand with imidazole and carboxylate donor moieties, viz., 2-(N-isopropyl-N-((1methylimidazolyl)methyl)aminomethyl)-6-(N-carboxylmethylN-((1-methylimidazolyl)methyl)aminomethyl)-4-methylphenol (ICIMP).22 The synthesis of the ligand is outlined in Scheme 1. An overall yield of 26%, based on 1-methylimi(13) Arnold, M.; Brown, D. A.; Deeg, O.; Errington, W.; Haase, W.; Herlihy, K.; Kemp, T. J.; Nimir, H.; Werner, R. Inorg. Chem. 1998, 37, 2920-2925. (14) Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 1175111757. (15) Barrios, A. M.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 91729177. (16) Konrad, M.; Meyer, F.; Jacobi, A.; Kircher, P.; Rutsch, P.; Zsolnai, L. Inorg. Chem. 1999, 38, 4559-4566. (17) Yamaguchi, K.; Koshino, S.; Akagi, F.; Suzuki, M.; Uehara, A.; Suzuki, S. J. Am. Chem. Soc. 1997, 119, 5752-5753. (18) Koga, T.; Furutachi, H.; Nakamura, T.; Fukita, N.; Ohba, M.; Takahashi, K.; Okawa, H. Inorg. Chem. 1998, 37, 989-996. (19) Meyer, F.; Pritzkow, H. Chem. Commun. 1998, 1917. (20) Volkmer, D.; Hoerstmann, A.; Griesar, K.; Haase, W.; Krebs, B. Inorg. Chem. 1996, 35, 1132-1135. (21) Volkmer, D.; Hommerich, B.; Griesar, K.; Haase, W.; Krebs, B. Inorg. Chem. 1996, 35, 3792-3803. (22) NMR 1H (CDCl3), δ (ppm): 1.14 (d, 6H, i-Pr), 2.19 (s, 3H, phenolMe), 3.07 (m, 1H, i-Pr), 3.33 (s, 2H, CH2CO2), 3.41 (s, 3H, CH3Imi-Pr), 3.63 (s, 3H, CH3Im-CO2), 3.74 (bs, 4H, CH2Ph), 3.82 (s, 2H, ImCH2-i-Pr), 3.84 (s, 2H, ImCH2-CO2), 6.80 (s, 1H, ImH-i-Pr), 6.81 (s, 1H, ImH-CO2), 6.84 (d, 1H, ImH-CO2), 6.89 (s, 1H, ImHi-Pr), 6.93 (s, 1H, Ph), 6.99 (s, 1H, Ph). FAB-MS m/z (rel intensity, %) 455 (MH+, 40), 302 (M - N(CH2MeIm)(i-Pr), 60), 286 (M N(CH2MeIm)(CH2CO2H), 60). Anal. Calcd for C24H34N6O3‚HCl: C, 58.70; H, 7.18; N, 17.12; Cl, 7.22. Found: C, 56.68; H, 7.11; N, 15.72; Cl, 5.98.
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COMMUNICATION Scheme 1. Synthesis of ICIMP
dazole-2-carbaldehyde, was obtained. ICIMP is a member of a rapidly growing group of asymmetric ligands that has been developed in recent years.20-21,23-25 To our knowledge, it is the first ligand of this kind to contain carboxylate donors. Addition of an ethanolic solution of nickel perchlorate to a mixture of ICIMP, diphenylacetic acid, and sodium methoxide dissolved in hot absolute ethanol (2Ni(II):1ICIMP: 2Ph2Ac:4NaOMe) leads to the formation of a green solution. After 1 h of stirring, a blue-green precipitate is slowly formed. Mass spectrometry indicates that the precipitate contains a tetranuclear complex, [Ni4(ICIMP)2(Ph2Ac)2]+, and microanalysis is consistent with the formulation [Ni4(ICIMP)2(Ph2Ac)2][ClO4]2 (1).26 The complex could be recrystallized by dissolving it in DMF and allowing tertbutyl methyl ether to diffuse into the solution. The resulting blue crystals of [Ni4(ICIMP)2(Ph2Ac)2(DMF)2][ClO4]2‚ 2.5DMF (2)27 exhibited a mass spectrum similar to that of 1. Compound 2 was also characterized by microanalysis and single-crystal X-ray diffraction.28 If the supernatant from the above-mentioned reaction in ethanol is incubated with urea (2Ni(II):1.2urea), a second tetranuclear complex which contains coordinated urea, [Ni4(ICIMP)2(Ph2Ac)2(urea)(H2O)][ClO4]2, is formed. Single blue crystals of [Ni4(ICIMP)2(Ph2Ac)2(urea)(H2O)][ClO4]2‚ 0.5EtOH‚H2O (3)29 could be grown, and its crystal structure has been determined. The molecular structures of 2 and 3 (Figure 2) are very similar and may be described as dimers of dimers.14,30 Each dimer consists of two nickel atoms which are bridged by one ICIMP ligand and one diphenylacetate. The vacant (23) Fenton, D. E.; Okawa, H. Chem. Ber./Recl. 1997, 130, 433-442. (24) Hommerich, B.; Schwoeppe, H.; Volkmer, D.; Krebs, B. Z. Anorg. Allg. Chem. 1999, 625, 75-82. (25) Lambert, E.; Chabut, B.; Chardon-Noblat, S.; Deronzier, A.; Chottard, G.; Bousseksou, A.; Tuchagues, J.-P.; Laugier, J.; Bardet, M.; Latour, J.-M. J. Am. Chem. Soc. 1997, 119, 9424-9437. (26) FAB-MS m/z (rel intensity, %): 1662 (Ni4(ICIMP)2(Ph2Ac)2ClO4, 40), 779 (Ni2(ICIMP)(Ph2Ac), 100). ES-MS m/z (rel intensity, %): 1662.1 (Ni4(ICIMP)2(Ph2Ac)2ClO4, 40), 779.3 (Ni2(ICIMP)(Ph2Ac), 100). Anal. Calcd for C80H98Cl2N12Ni4O20: C, 51.84; H, 5.33; N, 9.07. Found: C, 48.53; H, 4.95; N, 9.43. (27) FAB-MS m/z (rel intensity, %): 1662 (Ni4(ICIMP)2(Ph2Ac)2ClO4, 40), 779 (Ni2(ICIMP)(Ph2Ac), 100). Anal. Calcd for C89.5H117.5Cl2N16.5Ni4O22.5: C, 51.43; H, 5.67; N, 11.06. Found: C, 51.13; H, 5.22; N, 10.22. (28) Crystal data: C179H235N33O45Cl4Ni8, M ) 4180.33, triclinic, a ) 16.6097(3) Å, b ) 16.8816(4) Å, c ) 20.5384(5) Å, R ) 76.4019(15)°, β ) 79.1574(16)°, γ ) 61.2237(9)°, V ) 4887.23(19) Å3, T ) 120 K, space group P1h (No. 2), Z ) 2, µ(Mo KR) ) 0.71073 Å, 32713 reflections measured, 16032 unique reflections (Rint ) 0.0502). The final R value was 0.0573 and Rw(F2) 0.1277 for I > 2σ. Corresponding values for all data: R ) 0.0893 and Rw(F2) ) 0.1461.
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coordination sites of each Ni2 unit are filled by the carboxylate group of the ICIMP ligand of the opposite Ni2 moiety and one molecule of solvent or substrate (i.e., urea, cf. Figure 2). In complex 2, the additional ligands are two DMF molecules, while one urea molecule and one water molecule are coordinated in complex 3. Coordination of urea has previously been established for several model complexes for urease.13-19 An interesting feature of the general structure of the compounds is the asymmetric arrangement of the interaction between the two Ni2 units. In one Ni2 unit, the ligand carboxylate group from the neighboring dimer bridges the two nickel atoms, while in the second dimer, the corresponding carboxylate moiety chelates one nickel (Figure 2c). Thus, the two Ni2 units that are found in each tetranuclear complex represent two structural isomers that are related via a “carboxylate shift”.31 The coordination environments of each Ni2 unit in 2 and 3 are closely related to that of the active site of urease. The polydentate ligand provides coordinating moieties to mimic all the coordinating amino acid residues in the active site of urease except for the carbamylated lysine, which in each Ni2 unit is modeled by diphenylacetic acid. The carboxylate from the neighboring Ni2 moiety coordinates at the sites occupied by water molecules in the crystal structures of the enzyme (cf. Figure 1). All nickel atoms have N2O4 coordination, and one nickel atom in each dimer has a loosely bound solvent molecule, which indicates a possible site for urea coordination. As in the crystal structure of Bacillus pasteurii urease,9 the bridging groups are one carboxylate and one OH/OR moiety. In 2 and 3, the “bridging” dimer [Ni(1), Ni(2)] is resemblant of the active site of urease with respect to the nature of the ligand coordination. Here, the open coordination site is located on the nickel atom that does not have carboxylate coordination from its own ligand (cf. Ni1 in the B. pasteurii crystal structure, Figure 1). In 3, urea binds via (29) FAB-MS m/z (rel intensity, %): 1662 (Ni4(ICIMP)2(Ph2Ac)2ClO4, 30), 779 (Ni2(ICIMP)(Ph2Ac), 100). Anal. Calcd for C78H97Cl2N14Ni4O21.5: C, 49.82; H, 5.20; N, 10.43. Found: C, 49.36; H, 5.22; N, 9.46. Crystal data: C75H181Cl6N12Ni6O12, M ) 2008.28, triclinic, a ) 13.9016(2) Å, b ) 15.5577(2) Å, c ) 21.0623(3) Å, R ) 77.4506(5)°, β ) 89.7781(5)°, γ ) 71.9627(6)°, V ) 4218.06(10) Å3, T ) 120 K, space group P1h (No. 2), Z ) 2, µ(Mo KR) ) 0.71073 Å, 63001 reflections measured, 16412 unique reflections (Rint ) 0.0448). The final R value was 0.0566 and Rw(F2) 0.1483 for I > 2σ. Corresponding values for all data: R ) 0.0763 and Rw(F2) ) 0.1639. (30) Adams, H.; Clunas, S.; Fenton, D. E. Chem. Commun. 2002, 418419. (31) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15, 417-430.
COMMUNICATION
Figure 2. (a) ORTEP representation of compound 2. Selected interatomic distances and angles: Ni(1)-Ophenolate 2.012(3), Ni(1)-OPh2Ac 2.044(3), Ni(1)-ODMF 2.092(3), Ni(2)-Ophenolate 2.013(3), Ni(2)-OPh2Ac 2.069(3), Ni(3)-Ophenolate 2.012(3), Ni(3)-OPh2Ac 2.004(3), Ni(3)-ODMF 2.144(3), Ni(4)Ophenolate 2.008(3), Ni(4)-OPh2Ac 2.024(3), Ni(1)-Ni(2) 3.471, Ni(3)-Ni(4) 3.513, Ni(1)-Ni(3) 5.842, Ni(2)-Ni(3) 3.811, Ni(2)-Ni(4) 4.039, Ni(1)Ophenolate-Ni(2) 119.12(14), Ni(3)-Ophenolate-Ni(4) 121.81(15), Ni(1)-Ni(2)-Ni(3)-Ni(4) 145.6. (b) ORTEP representation of compound 3. Selected interatomic distances and angles: Ni(1)-Ophenolate 1.992(3), Ni(1)-OPh2Ac 2.055(3), Ni(1)-Ourea 2.102(3), Ni(2)-Ophenolate 2.010(3), Ni(2)-OPh2Ac 2.074(3), Ni(3)-Ophenolate 2.006(3), Ni(3)-OPh2Ac 2.004(3), Ni(3)-OH2O 2.184(3), Ni(4)-Ophenolate 2.002(3), Ni(4)-OPh2Ac 2.011(3), Ni(1)-Ni(2) 3.468, Ni(3)-Ni(4) 3.492, Ni(1)-Ni(3) 5.757, Ni(2)-Ni(3) 3.794, Ni(2)-Ni(4) 4.004, Ni(1)-Ophenolate-Ni(2) 120.13(13), Ni(3)-Ophenolate-Ni(4) 121.19(13), Ni(1)-Ni(2)Ni(3)-Ni(4) 150.9. (c) Schematic representation of the connectivity in compounds 2 and 3.
its carbonyl oxygen to the open coordination site in the bridging dimer. This suggests that the initial step in the catalytic cycle of urease is the coordination of the urea carbonyl to Ni1 in the enzyme. The “chelated” dimer [Ni(3), Ni(4)] shows similarities to the B. pasteurii urease in that it has the same number of bridges between the nickel centers, which makes the Ni-Ni distances comparable to those found in urease.7,9 The Ni-Ni distance is 3.51 (2), 3.49 (3) Å in the chelated Ni2 unit vs 3.47 Å (2 and 3) in the bridged unit. In order to assess the capacity of these compounds to act as functional models for hydrolytic metalloenzymes,24 their ability of catalyzing the hydrolysis of organophosphate esters is currently being studied.32 Complex 1 is a suitable candidate for a hydrolysis catalyst, as it is believed to possess vacant coordination sites (or loosely bound solvent molecules) where suitable substrates may bind. The rate of internal hydrolysis of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP)33 at pH 8.0 and 25° was increased by a factor of 21 in the presence
of 1, compared to the uncatalyzed reaction (5.1 × 10-6 M/h vs 2.4 × 10-7 M/h). Relatively moderate turnover numbers (10-15) were recorded for the reaction, possibly due to inhibition by the phosphate product. The fact that 1 catalyzes hydrolysis despite the lack of a bridging hydroxide might indicate that such a moiety is not crucial to urease catalysis.10 A dinuclear compound that is tentatively assigned the formula [Ni2(ICIMP)(Ph2Ac)2]34 may be synthesized by repeating the above-mentioned reaction using an excess of diphenylacetic acid; however, attempts to cleave the tetranuclear compounds into dinuclear complexes by reaction with diphenylacetic acid have thus far proven unsuccessful.
(32) Albedyhl, S.; Averbuch-Pouchot, M. T.; Belle, C.; Krebs, B.; Pierre, J. L.; Saint-Aman, E.; Torelli, S. Eur. J. Inorg. Chem. 2001, 14571464. (33) Brown, D. M.; Usher, D. A. J. Chem. Soc. 1965, 6558-6564.
IC025839P
Acknowledgment. This research is supported by grants from the Swedish Research Council (VR, to E.N.) and the Academy of Finland (to M.H.). Supporting Information Available: Supporting material concerning crystal structures and details concerning the hydrolysis reaction (CIF and PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
(34) FAB-MS m/z (rel intensity, %): 1013 (Ni2(ICIMP)(Ph2Ac)2Na, 65), 779 (Ni2(ICIMP)(Ph2Ac), 100).
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